This invention relates to methods and apparatus for monitoring the condition and status of high voltage bushings used in conjunction with power transformers and, in particular, to methods and apparatus for doing so safely.
Power transformers are one of the principal elements of the power system and bushings are an important component of transformer equipment. As shown in FIG. 1, high voltage bushings 10 are coupled and located about the high power transformer terminals. The bushings generally include specially designed electrical terminals for taking out winding ends (leads) through openings provided in the cover or wall of transformer tanks and connecting to incoming and outgoing lines. The bushings act as insulators to prevent a short circuit or “arcing”. In large power transformers, the voltages used are so high that the wires cannot be allowed to come too close to each other, or too close to the metal casing of the transformer. If they do get too close, then the voltage can actually jump through the air (electrical breakdown), and create a short circuit. Bushings are therefore an important element in the reliable operation of their associated transformers. On a global scale, defects of bushings reportedly cause from 10% to 40% of the total number of failures of power transformers.
Bushings used in electrical distribution and transmission systems enable high voltages (e.g., from less than 69 kVAC to more than 765 kVAC) to be connected to devices such as transformers and circuit breakers. High voltage bushings of interest include capacitive layers to provide voltage grading across the bushing. The capacitance may range from less than 200 to more than 2,000 Pico farads. As shown in FIG. 1A, the capacitance of a bushing 10 may be represented as being split into two separate capacitors: C1 and C2; with C1 representing the capacitance between the test tap 13 and the high voltage point, or power terminal, 11 and C2 representing the capacitance between the test tap 13 and terminal 15 which is grounded. One side of C1 is the draw rod (lead) 11 which is the conductive lead to which the high voltage (HV) is applied and which passes through the bushing and the other side of capacitor C1 is connected to test tap 13. One side of the second capacitor C2 is connected to the test tap 13 and the other side of C2 is connected to ground 15. These capacitors may be constructed by using a conductive film on an insulating substrate (layer) that in many cases is paper. This insulating layer may be liquid-immersed typically in mineral oil; but, other insulating mediums could be used.
Over time the bushing and its associated capacitor layers may begin to degrade due to a faulty seal which allows ambient air or other contaminants to mix in with the insulating fluid. Also, moisture can accelerate the degradation of the bushing and its capacitive layers. As the capacitive layer begins to fail, there will be tracking on the paper layer due to evolving partial discharge.
Eventually, the C1 capacitor can short from the draw rod 11 to the bushing test tap 13. Should this occur, the line voltage which may range from 69 kV to 765 kV will be seen at the bushing test tap 13. While many existing bushing monitor couplers contain built in surge arrestors, should the surge arrestors fail or become inoperative, the full bushing potential will appear at the measurement hardware. Under this scenario the measurement electronics will be badly damaged possibly causing a localized fire. Even worse, there is the possibility of a serious or even a fatal injury should personnel be near the affected equipment.
A proposed solution to the problem using an isolation transformer to inductively couple the output of the bushing coupler to a monitoring system does not solve the problem. This may be illustrated by reference to a testing scheme disclosed in U.S. Pat. No. 6,927,562 titled Power Factor/Tan(δ) Testing Of High Voltage Bushings on Power Transformers, Current Transformers and Circuit Breakers. FIG. 2 of the U.S. Pat. No. 6,927,562 is redrawn as FIG. 2 (Prior Art) of the instant application. In the event of the failure of capacitor 18c, the full line voltage will appear at a center tap 22c. If the surge arrestors in the bushing coupler are inoperative, the full line voltage (E1/I1) will appear across the primary of transformer 42 as shown in FIG. 2A which is derived from FIG. 2. Transformer 42 would then have to sustain the entire voltage seen at the test tap. This could destroy the transformer 42 and, even if it does not destroy the transformer 42, an inordinately large and potentially dangerous voltage would be produced at the output side of transformer 42 destroying any testing or monitoring equipment (in box 32) coupled thereto and endangering the life of individuals operating the equipment.
It is an object of the invention to couple the test tap of a bushing to a bushing coupler whose output is transmitted to testing and monitoring equipment such that the testing and monitoring equipment and any operator of the equipment has no physical contact to the bushing coupler and/or the test tap of the bushing.